1. Field of the Invention
The present invention relates to an ink jet head that performs recording by discharging ink onto a recording medium.
2. Related Background Art
Because producing high quality characters and images is easy with ink jet recording apparatuses, such output devices are widely employed today, especially for computers. Above all, bubble jet systems, wherein ink is forcefully discharged from nozzles by utilizing extremely powerful pressure changes produced by the instantaneous boiling of ink in the nozzles, have become the leading, preferred ink jet recording apparatuses.
Further, as the popularity of ink jet recording apparatuses has grown, so too has the number of requests for improved performance, especially as it pertains to image quality and recording speeds. And since to improve image quality, the diameters of dots formed on a recording medium (specifically, on a recording sheet) are especially important, greater emphasis is placed on the provision of smaller dot diameters for the recording of images, such as photographs, than for the recording of characters. For example, to produce clear, eye-pleasing, or small, characters when recording documents, resolutions ranging from 600 to 1200 dots per inch (dpi) are required, and to provide satisfactory dot diameters, droplets of 80 to 90 μm (about 30 pl, as volume) must be discharged.
On the other hand, for image recording, a resolution of 1200 to 2400 dpi is required to provide smooth tones equivalent to those in a silver halide photograph. Thus, for recording processes performed at these resolutions, when the dot diameter of a droplet to be discharged is 40 μm (about 4 pl, as volume), it is required that two types of ink, having dye densities that differ and ratios of about 1/4 to 1/6, be separately employed, depending on the image density. Whereas when the dot diameter of a droplet to be discharged is reduced to 20 μm (about 0.5 pl, as volume), only one type of ink having a single density need be employed to obtain both the acceptable density for a high density portion and the desirable smoothness for a low density portion. As is described above, reducing the sizes of the droplets that are discharged is required in order to secure the same image quality as that provided by a silver halide photograph.
However, when the sizes of the discharged droplets are reduced, an increased number of dots is required to form an image. For example, to fill an area of 8 inches (about 20 cm)×11 inches (about 28 cm), which is substantially the same size as A4 stock, 130 million 4 pl dots would suffice, while for the same area 250 million 2 pl dots, 500 million 1 pl dots or 1 billion 0.5 dots would be required.
Further, to maintain recording speed while the sizes of the droplets that are discharged are reduced, a corresponding increase in the discharge frequency is required. In this instance, to increase the discharge frequency, an ink volume equivalent to that discharged as droplets from the nozzles of a recording head must be rapidly supplemented from a source upstream of the nozzles, and to implement this, a low nozzle flow resistance is needed (i.e., in cross section, a large flow path is required).
FIGS. 9A and 9B are a plan view and a cross-sectional view for explaining the positional relationship of an ink flow path, a heat generating element and a discharge port in a conventional ink jet head for discharging small droplets.
The conventional ink jet head comprises: a substrate 1001, on the surface of which multiple heat generating elements 1004 are mounted for boiling ink and generating bubbles; and a flow path formation member 1003, for forming, with the substrate 1001, ink flow paths 1002 corresponding to the heat generating elements 1004. The flow path formation member 1003 includes partition walls 1003a for defining the ink flow paths 1002, and a ceiling wall 1003b, provided on the partition walls 1003a parallel to the substrate 1001. Discharge ports 1005 are formed in the ceiling wall 1003b, centrally arranged above the individual heat generating elements 1004, so that ink is discharged by the pressure exerted when the heat generating elements 1004 produce bubbles. In order to reduce the size of a droplet to be discharged, it is preferable that the size of the heat generating element 1004 be reduced in proportion to the volume of the droplet, while taking the improved energy efficiency into account. Generally, the size of a bubbling chamber is reduced in accordance with the size of the heat generating element. However, when the heat generating elements are arranged at pitches of 600 dpi or higher, for example, and when, in the conventional manner, the bubbling chamber is reduced in accordance with the capabilities of the heat generating element, the flow resistance in the nozzles will become too high and a desired discharge frequency will not be obtained. Therefore, when the size reduction ratio of the bubbling chamber to the heat generating element is set so it is smaller than the conventional ratio, i.e., relative to the heat generating element, the size of the bubbling chamber is larger than the conventional one, the size of the flow path can be increased in cross section, and the desired discharge frequency can be obtained. Actually, since the discharge characteristic may be changed greatly by changing the height of the flow path, mainly the width of the ink flow path 1002 is increased to obtain the desired discharge frequency.
However, when, as is shown in FIG. 9B, compared with the size of the heat generating element 1004 the width of the flow path 1002 is satisfactorily large, a greater stagnated ink portion is generated near the corners formed by the partition walls 1003a and the ceiling wall 1003 that define the ink flow path 1002. And in the stagnated ink portion, residual bubbles, retained in the ink, absorb discharge pressure exerted during the bubbling process and prevent a preferable ink discharge operation from being performed.